Please do look into this bug as it affects a wide range of hardware and a lot of users as is evidenced in the KDE bug report.
If more information is required to implement or find the proper resolution, I will try to provide what I can.

The code posted applies to mesa 7.8, but has been removed in 7.9. It would be useful to
a) Test if this bug still applies to mesa 7.9 from git (I've been told it applies to a snapshot from 2010/09/09, but haven't been able to reproduce it myself).
and if it still occurs, then
b) Port the patch to the 7.9 branch & master.

(In reply to comment #3)
> The code posted applies to mesa 7.8, but has been removed in 7.9. It would be
> useful to
> a) Test if this bug still applies to mesa 7.9 from git (I've been told it
> applies to a snapshot from 2010/09/09, but haven't been able to reproduce it
> myself).
>
> and if it still occurs, then
> b) Port the patch to the 7.9 branch & master.
Thanks for looking at this. So, do I understand correctly, that the patch is ok on Mesa-7.8.x ? If this is so, then everyone now using KDE-4.5.x (and presumably Mesa-7.8.x), can use it.
I'd love also to test it out on 7.9 -- but I tried several weeks ago and ran into trouble -- I think with the intel driver (xf86-video-intel-2.12.0). I'll try again and get back to you on this.
Have you or anyone had a chance to look at BUG #30217 ?
thanks again

An update with some good news regarding comment #3:
I did further testing with the following setup:
Intel Corporation 82915G/GV/910GL Integrated Graphics Controller (i915)
xorg-server-1.9.0, xf86-video-2.13.0, libdrm-2.4.22, mesa-7.9-rc2, qt-4.7.0
kde-4.5.1, linux-2.6.35.7
As I stated in comment #5, with this build, desktop (GLX) effects will not activate. There's a kde desktop effect called 'blur' which is activated on first kde-desktop boot on my system, but has never worked on my system. Up to now, I've just left it on as it doesn't appear to cause any problems. However, with this new build, 'blur' is in fact the culprit: if I first manually turn off this effect then GLX effects are activated, and the Pixmap destroy issue does not occur.
Conclusion: Problem exists in Mesa-7.8.2/3rc1, but has been fixed in Mesa-7.9rc2.
The unrelated 'blur' issue, at least for the moment, is kde's problem...
thanks for your time.